Endometriosis symptons can occur because the uterus is located in the pelvic cavity and carries the baby during pregnancy. The lining of the uterus consists of a type of tissue called the endometrium – composed of endometrial cells that provide a safe environment for a baby to grow.
Prior to pregnancy each month, the endometrium thickens in preparation for the fertilsation of an egg. When the egg isn’t fertilised, the endometrium breaks down and exits the body during the menstrual period.
In women with endometriosis, some of this endometrial tissue flows back through the fallopian tubes and then starts to grow outside the uterus.[i]
Endometrial cells that grow outside the uterus are called implants and typically grow on the ovaries, fallopian tubes, outer wall of the uterus or the intestines. The body can’t naturally expel the implants, even though they’re the same tissue that lines the uterus as it is trapped in the pelvic area and lower stomach (abdomen) and sometimes very rarely in other areas in the body.
With nowhere for the displaced endometrial tissue to escape from, the tissue instead bleeds into the pelvic cavity, becoming fibrous over time and eventually causing scar tissue.